1. Technical Field
Embodiments of the present invention relate to a sheet feeder and an image forming apparatus incorporating the sheet feeder.
2. Related Art
As an example of a sheet feeder that can be incorporated in the image forming apparatus, an electrostatic attraction/separation system has been proposed as a method of separating and conveying a sheet such as an original document and a recording medium loaded on a sheet tray. The electrostatic attraction/separation system generates an electric field on an attraction belt so that the sheet contacts the attraction belt and then separates from the attraction belt.
Japanese Patent Application Publication No. JP 2012-056711-A discloses a sheet feeder having the electrostatic attraction/separation system. The sheet feeder includes an attraction/separation unit including a dielectric attraction belt that is wound about two rollers, an electric charge applying unit that serves as an attraction unit to apply an alternating electric charge to the attraction belt, and a holder that holds the dielectric attraction belt and the electric charge applying unit. The holder rotatably supports the two rollers and is fixed to a rotary shaft that is disposed upstream from the two rollers in a sheet feeding direction. Further, the sheet feeder includes a swing unit to swing the attraction/separation unit about the rotary shaft so that the attraction belt reciprocally moves between a sheet contact position and a sheet separation position. The sheet contact position is a position at which the attraction belt contacts and attracts an uppermost sheet of a sheet stack loaded on a bottom plate of a sheet tray. The sheet separation position is a position away from the sheet contact position and where the uppermost sheet attracted to the attraction belt separates from the sheet stack to be conveyed for a subsequent image forming operation.
The two rollers are an upstream roller and a downstream roller.
The upstream roller is disposed upstream from the downstream roller in the sheet feeding direction and supported by the holder. Specifically, when the attraction/separation unit is moved from the sheet contact position to the sheet separation position, the upstream roller is supported by the holder rotatably within a given range such that the upstream roller continues to contact the upper surface of the sheet stack until the attraction/separation unit swings by a given angle and separates from the sheet stack together with the holder when the attraction/separation unit swings by a greater angle than the given angle of inclination.
By contrast, the downstream roller is disposed downstream from the upstream roller in the sheet feeding direction. Specifically, when the attraction/separation unit is moved from the sheet contact position to the sheet separation position, the downstream roller is supported by the holder so as to separate from the sheet stack together with the holder from the start of movement of the attraction/separation unit.
Prior to a sheet feeding operation, the attraction belt that is supported by the holder via the upstream roller and the downstream roller remains separated from the sheet stack. When the uppermost sheet is separated from the sheet stack to convey, the attraction belt is rotated before being applied with an alternating electric charge. The alternating electric charge is uniformly applied to the attraction belt, rotation of the attraction belt is stopped. Thereafter, the swing unit is driven to swing the attraction/separation unit toward the sheet stack. Then, the attraction belt contacts the uppermost sheet of the sheet stack, so that the uppermost sheet of the sheet stack is attracted to the attraction belt. At this time, the upstream roller is released from the holder and placed on the upper surface of the sheet stack.
When the uppermost sheet of the sheet stack is attracted to the surface of the attraction belt placed on the upper surface of the sheet stack, the swing unit is driven to swing the attraction/separation unit from the sheet contact position to the sheet separation position. When swing of the attraction/separation unit from the sheet contact position to the sheet separation position starts, the downstream roller moves in a direction to separate from the sheet stack together with the holder. By contrast, the upstream roller remains under its own gravity on the upper surface of the sheet stack with the attraction belt interposed therebetween. Accordingly, a downstream surface of the attraction belt downstream from the upstream roller in the sheet feeding direction is inclined with respect to the upper surface of the sheet stack. Therefore, a part of the uppermost sheet attracted to the surface of the attraction belt is lifted while being bent about a nip portion on the uppermost sheet pressed by the upstream roller with the attraction belt therebetween serving as a pivot. Thereafter, the upstream roller is lifted by the holder and moves together with the holder to separate from the upper surface of the sheet stack and move to the sheet separation position. When the attraction/separation unit reaches the sheet separation position, the attraction belt is rotated to convey the uppermost sheet that is attracted to the attraction belt.
By bending the uppermost sheet about the nip portion pressed by the upstream roller while sandwiching the attraction belt, a subsequent sheet attached to the uppermost sheet due to an adhesion force separates by the force of gravity from the uppermost sheet. When separating from the upper surface of the sheet stack, an angle of inclination of the downstream surface of the attraction belt and the upper surface of the sheet stack is different according to rigidity of sheet. A sheet having a high rigidity separates with a relatively small angle of inclination of the attraction belt while a sheet having a low rigidity separates with a relatively large angle of inclination of the attraction belt. If the large angle of inclination of the attraction belt for separating the low-rigidity sheet is employed for separating the high-rigidity sheet, even the uppermost sheet separates under its own rigidity from the attraction belt.
To address the inconvenience, the sheet feeder disclosed in JP 2012-056711-A provides different angles of inclination of the attraction belt with respect to the upper surface of the sheet stack according to sheet rigidity when the upstream roller separates from the sheet stack. Specifically, the sheet feeder disclosed in JP 2012-056711-A includes a unit to change a range of movement of the upstream roller with respect to the holder according to sheet rigidity. With this unit, as the rigidity of a sheet to be conveyed increases, the range of movement of the upstream roller with respect to the holder decreases. Therefore, as the rigidity of sheet to be conveyed increases, the upstream roller can be lifted by the holder with a small angle of inclination of the attraction belt. Accordingly, when handling a sheet having a large rigidity, separation of the uppermost sheet from the attraction belt can be prevented.
As described above, by changing the angle of inclination of the attraction belt according to sheet rigidity when separating the upstream roller from the sheet stack, the sheet feeder disclosed in JP 2012-056711-A can obtain a good separation performance regardless of various sheet rigidities.
However, when the sheet feeder disclosed in JP 2012-056711-A feeds a thin paper having a low rigidity, the uppermost sheet occasionally does not separate from a subsequent sheet of the sheet stack. After the research and study of the problem, it was found that sheet separation is significantly affected by a curvature of a curved part of the sheet than an angle of inclination of the attraction belt. Specifically, by increasing the curvature of the curved part of the sheet and bending the sheet more tightly as rigidity of the sheet decreases, the uppermost sheet can separate from the subsequent sheet reliably. Since the sheet feeder disclosed in JP 2012-056711-A bends the sheet due to the curvature of the upstream roller, when a thin paper having a low rigidity is used, the subsequent sheet cannot be separated from the uppermost sheet.
To increase the curvature of a curved part of the sheet, a diameter of the upstream roller can be decreased. However, even a thick paper having a large rigidity is bent tight by the upstream roller having the smaller diameter. Accordingly, it is likely that the uppermost sheet also separates from the attraction belt due to the rigidity of the uppermost sheet.